We found that beta-glucuronidase increases in activity beginning at the 2-cell stage in mouse embryos. Between the eight-cell stage and early blastocyst, 60 through 84 hours post fertilization, there is a 100 fold increase in beta-glucuronidase activity. By using genetic variants of the glucuronidase locus we have determined that paternal genes are expressed in this increase which indicates that embryonic genes are transcribed and translated by the 2-cell stage of development. Preliminary findings indicate that two additional acid hydrolases, beta-galactosidase and alpha-galactosidase, also increase in activity during preimplantation development. I plan to extend these studies to determine (1) whether these enzymes increase coordinately with each other during early development and (2) whether embryonic genes for alpha-galactosidase and beta-galactosidase are responsible for the increase in activity. In the later instance we will utilize genetic variants of the structural genes and determine embryonic gene expression by looking for paternal gene expression. Following the characterization of the increase in activity of these three enzymes we will examine enzyme activity changes in various inbred strains to determine whether there is genetic variation for the increase in activity of individual enzymes during the preimplantation period. BIBLIOGRAPHIC REFERENCES: Wudl, Linda and Chapman, Verne, 1976. The expression of beta-glucuronidase during preimplantation development of mouse embryos. Developmental Biology 48, 104-109. Chapman, V.M., Adler, D., Labarca, C., and Wudl, L., 1976. Genetic variation of beta-glucuronidase expression during early embryogenesis in: McLaren, A. Embryogenesis in Mammals. Ciba Foundation Symposium, No. 40. Observer/Excerpta Media/North-Holland Associated Scientific Publishers, New York.